Derivative Instruments and Hedging Activities - Gains or Losses Related to Changes in Fair Value and Cash Settlements on Derivatives Not Qualifying as Hedging Instruments (Detail) - Not Designated as Hedging Instrument - Commodity contracts - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Cash settlements	$ 0.0	$ 4.1	$ 0.0	$ 7.2
Change in fair value	0.0	(4.4)	0.0	(5.3)
Total gains (losses) on non-qualifying hedges	$ 0.0	$ (0.3)	$ 0.0	$ 1.9